This invention relates to catalysts for removing organic compounds from a gas stream containing the same and processes for removing organic compounds from the gas stream using the catalysts, and particularly to such catalysts as are effective for removing organic compounds diluted in a gas stream and processes for removing such organic compounds from the gas stream using the catalysts.
A wide range of synthetic organic compounds are processed and produced as intermediates and products in chemical plants, with a concomitant leakage or accidental discharge into the environment. Frequently, exhaust gases from waste incinerator plants contain organic compounds. Many such organic compounds are harmful to both the environment and health. Consequently, there is a need to establish an effective emission control technique. Particularly, there is a need to develop effective and practical techniques for controlling emission of volatile organic compounds or halogen-containing organic compounds.
There have been proposed many methods for removing organic compounds from gas streams, including adsorption, direct combustion and catalytic combustion methods. Though the adsorption method may work effectively when it is employed for treating a gas stream having a high content of organic compounds, removal efficiency may deteriorate when it is used for treating a gas stream containing dilute organics. In the case of the direct combustion method, a temperature as high as 800° C. or higher is required, which is not economically viable. Furthermore, the direct combustion method suffers from the problem that nitrogen oxides are generated which could result in a source of secondary pollution.
As a catalytic combustion method, for example, Japanese Patent Public Disclosure (KOKAI) No. HEI-4-250825 describes a method for treating a gas stream containing halogen-containing organic compounds, wherein said gas stream is brought into contact with an acidic zeolite. In the method, the halogen-containing organic compounds are treated by contacting the gas stream with an acidic zeolite or an acidic zeolite that is loaded and/or exchanged with at least one metal belonging to the 2nd to 6th periods of the Periodic Table of Elements.
Further, Japanese Patent Public Disclosure (KOKAI) No. HEI-4-284849 discloses a catalyst for decomposing 1,2-dichloroethane and a method for treating a waste gas stream with such a catalyst. In the disclosed method, the 1,2-dichloroethane decomposing catalyst comprises an acidic zeolite or an acidic zeolite which has been loaded and/or ion-exchanged with a transition metal(s).
Further, Japanese Patent Public Disclosure (KOKAL) No. HEI-8-38896 discloses a catalyst for decomposing chlorinated volatile organic compounds. According to the disclosure, the catalyst is capable of decomposing chlorinated volatile organic compounds in the presence of steam and oxygen. The catalyst comprises at least one element selected from platinum, palladium and ruthenium as a primary catalytically active substance which is supported on a carrier consisting essentially of zirconia, said carrier supporting further a cocatalyst comprising boron oxide. In the catalyst, the proportion of the primary catalytically active ingredient is, calculated as metal, 0.1 to 5% by weight on the basis of the total weight of catalyst, and the proportion of the cocatalyst is, calculated as B2O3, 2 to 5% by weight on the same basis.
Japanese Patent Public Gazette (KOKOKU) No. HEI-6-87950 describes a process for catalytically reacting a waste gas stream containing hydrocarbons, halogenated hydrocarbons and carbon monoxide, and an apparatus for effecting such a process. The reference particularly describes a process for catalytically contacting a waste gas stream from vinyl chloride synthesis, wherein said gas stream is passed through a first zone including a catalyst for oxidatively decomposing the noxious compounds at 300–800° C., and then through a second zone including a catalyst for oxidatively combusting the noxious compounds. The first zone catalyst comprises, as a catalytically active substance, aluminum oxide, silica dioxide and/or a zeolite, which may contain optionally 0.1 to 20% by weight of one or more oxides of elements Ba, Cu, Cr and Ni. The second zone catalyst comprises, as a catalytically active ingredient, platinum and/or palladium, or a combination of platinum and rhodium.
As above-mentioned, prior art references Japanese Patent Public Disclosure (KOKAI) Nos. HEI-4-250825 and 4-284849 disclose methods for treating gas streams contaminated with halogen-containing organic compounds, in which the gas streams are brought into contact with either an acidic zeolite or an acidic zeolite supporting and/or exchanged with at least one metal belonging to the 2nd–6th periods of the Periodic Table of Elements. However, These catalysts are not satisfactory in their gas cleaning performance in that the methods can produce by-products including halogen-containing compounds other than those that are envisaged to be removed through the methods.
On the other hand, the combustion methods disclosed in the above-mentioned references Japanese Patent Public Disclosure (KOKAI) No. HEI-8-38896 and Japanese Patent Public Gazette (KOKOKU) No. HEI-6-87950 require a considerably raised operation temperature exceeding about 500° C.
Accordingly, one of the important objects of the invention is to provide improved high-performance combustion catalysts capable of treating a waste gas stream containing organic compounds without yielding undesirable by-products.
A further primary object of the invention is to provide a method in which such a catalyst is employed efficiently for removing harmful or noxious organic compounds present in a gas stream without yielding undesirable by-products. The other objects and advantages of the invention will become clear from the description given below.